Electroluminescent device



Jan. 12, 1965 c. H. HAAKE 3,

EILECTROLUMINE$CENT DEVICE Filed June 10, 1960 FIG. I.

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CONTINUOUS WAVE HIGH FREQUENCY GENERATOR PULSE GENERATO R INVENTOR.C0405 H. f/fl/IKE United States Patent 3,165,667 ELEQTRQLUMINEdQENTDEVl'JE Claus H. Haake, Livingston, N..l., assignor to WestinghouseElectric Corporation, East Pittsburgh, Pa.,' a corporation ofPennsylvania Filed June 10, 1966, Ser. No. 35,339 5 Claims. (Cl.315-169) This invention relates to electroluminescent devices and, moreparticularly, to a device comprising an elec troluminescent element andthe associated element energization means.

The phenomenon of electroluminescence was first disclosed by G.Destriau, one of his earlier publications appearing in London, Edinburghand Dublin Philosophical Magazine, Series 7, Volume 38, No. 285, pages700-737 (October 1947). Since this early publication, electroluminescentlamps have been commercially marketed as light sources. Theelectric-field-responsive characteristics of electroluminescence hasmade possible many various types of image-presentation andimage-intensification devices. An early type of electroluminescentdevice which operates as an image intensifier is disclosed in U.S.Patent No. 2,650,310, dated August 35, 1954. A cross-grid or so-calledX-Y plotter electroluminescent image presentation apparatus is disclosedin U.S. Patent No. 2,698,915, dated I anuary 4, 1955. Many differentconstructions for signal presentation devices are known. In theoperation of an X-Y plotter, for example, individual signals are placedacross preselected cross grids and the electroluminescent phosphor whichis positioned thereoetween emits light in response to the generatedelectric field. In order to present a moving image, it is necessary toapply pulses of energization potential in a predetermined and rapidfashion to the individual lead conductors which comprise the crossgrids. For some applications of such devices, it is desirable to be ableto minimize as much as possible the magnitude of the potential pulseswhich are required for energization. In addition, it is desirable insome X-Y plotter applications to minimize the light emission decaycharacteristics of the electroluminescent phosphor material, so that afast moving image can be presented with greater clarity.

It is the general object of this invention to provide anelectroluminescent device Which'is adapted to be operated with a pulsedenergization and wherein a smaller potential magnitude can be used forthe exciting pulses, while still achieving a high level of light output.

It is another object to provide an electroluminescent device adapted tobe energized with a pulsed excitation, wherein the phosphor light outputwill decay in a rapid fashion after the energizing potential is removed.

The aforesaid objects of the invention, and other objects which willbecome apparent as the description proceeds, are achieved by providing acombination of electroluminescent element or elements and the associatedenergization means. The electroluminescent element is adapted to beexcited to light emission by a first potential generator which deliversa continuous, high-frequency potential across the device electrodes anda second potential generator which simultaneously delivers potentialpulses across the spaced electrodes. The continuous excitation and thepulsed excitation potentials are controlled relative to one another withrespect to their frequencies, repetition rates and relative magnitudes,in order that the 3,165,567 Patented Jan. 12, 1965 ice resulting lightoutput will be enhanced and the light output decay characteristics forthe phosphor will be more rapid.

For a better understanding of the invention, reference should be had tothe accompanying drawing wherein:

FIG. 1 is a sectional elevational view of an electroluminescent elementwith the associated energization means therefor shown in block diagram;

FIG. 2 -is a perspective view of a'por-tion of an XY plotter adapted tobe energized with both a continuous high frequency potential and apulsed potential, in accordance with the present invention.

With specific reference to the form of the invent-ion illustrated in thedrawing, the electroluminescent element ill as shown in FIG. 1 generallycomprises a glass foundation 12 having coated thereover a firstelectrode 14 which can be formed of tin oxide. Coated over theelectrode-l4 is a layer 16 comprising electroluminescent phosphor andcoated over the layer 16 is a second electrode 18 which can be formed ofvacuum-metallized aluminum or copper iodide for example. Connected tothe element electrodes 14 and 18 are a continuous-wave, high-frequencypotential generator 20 and a pulsed potential generator 22, bothpotential generators being shown in block diagram. The potential sources20 and 22 desirably are selected to have a high impedance and areconventional in design. The respective performance characteristics ofthese sources will be considered in detail hereinafter. The manner inwhich the sources 2% and 22 are connected to the electrodes 14 and 18 isimmaterial.

Any electroluminescent phosphor can be used in forming the layer 16 and,as an example, a green-emitting zinc sulfide phosphor activated bycopper and coactivated by chlorine has been found to be very suitable.The phosphor is desirably mixed with a dielectric such as equal parts byweight of phosphor and polyvinyl-chloride acetate dielectric material.Alternatively, the phosphor can be included between the spacedelectrodes without any admixed dielectric or a separate layer ofdielectric material can be included between the electrodes 14 and 18.The thickness of the layer 16 is not critical and can be variedconsiderably and a thickness of two mils has been found to be verysatisfactory. Either or both of the electrodes 14 and 18 can be replacedby a mesh of wires, such as can be used in a conventional X-Y plotter.Alternatively, the electrodes can be replaced by an interlacing mesh ofwires with phosphor therebetween, as disclosed in U.S. Patent No.2,684,450, dated July 20, 1954. For some special applications, it isdesirable to include other material layers between the electrodes 14 and1-8, such as a layer of photoconductive material if the device is to beused as a light amplifier. Alternatively, an additional layer ofnon-linear resistive material can be included between the electrodes 14and 18 in order to increase the sensitivity of the element 10 to respondwith greatly in creased brightness when excited by relatively intenseelectric fields. The element lit, as shown in FIG. 1 thus comprisesspaced electrodes having electroluminescent phosphor includedtherebetween. The element Ill can be of various size or configurationdepending upon its intended application.

In FIG. 2 is shown a portion of an X-Y plotter 24 which comprises afirst set or grid of parallel conductive strips 25 and a closely spacedsecond set or grid of parallel conductive strips 23. In the embodimentas amass? shown, at least one set of the conducting strips 26 or 28 islight transmitting. The conductors 26 and 2S comprising the individualgrids are disposed at an angle with respect to one another, andpreferably at approximately right angles. Material comprisingelectroluminescent phosphor Si) is included between contiguous portionsof the conductive strips 26 and 23 comprising the individual grids. As amatter of manufacturing convenience, the phosphor 3%) will normally beformed as a continuous layer. In the device 24, each of the contiguouscrossing points of each of the conductors 26 and 28 and the phosphorthcrebetwecn essentially comprise an electroluminescent element, such asdisclosed in FIG. 1. The cross grids are adapted to be energized throughsuitable synchronized commutating means 32, which distribute a part ofthe energizing potential to predetermined conducting strips comprisingthe grids, in order to present a predetermined composite image. Such adevice and the commutation therefor is generally disclosed inheretoforenlentioned US. Patent No. 2,698,915.

In the modification of the XY plotter 24 in accordance with the presentinvention, the continuous output of the potential generator 29 isapplied at all times across each of the conductive strips 26 and 28. Thepulsed output of the potential generator 22 is commutated anddistributed across the conductor 26 and 23 in predetermined fashion sothat the enhanced light output obtained through the application of thecontinuous potential and the pulsed potential produces the desired,predetermined visible signal. A conventional video input 34 controls thevoltage of the pulses in accordance with the predetermined signaldesired to be displayed.

Both of the electroluminescent combinations as shown in FIGS. 1 and 2generally comprise an electroluminescent element or plurality ofelectroluminescent elements and the associated energization meanstherefor. The element or elements each comprise spaced electrodes withelectrolurninescent phosphor included between the spaced electrodes andthe energization means comprises a continuous-wave, high-frequencypotential generator and a pulsed potential generator. As will bedescribed in greater detail hereinafter, when a predetermined continuouspotential and pulsed potential are applied across the electrodes, theresulting electroluminescent light output is considerably enhanced. Inaddition, the decay in electroluminescent output is faster by a factorof about 30% than would be encountered if only the exciting pulses wereto be applied across the electrodes. In order to realize the enhancedlight output and more rapid decay, the continuous alternating potentialshould have a frequency of at least 10 kilocycles and desirably shouldbe considerably higher. Also, in order to minimize the backgroundlighting effects of the continuous potential, the magnitude of thepotential of the pulses should be at least as great as the magnitude ofthe continuous alternating potential. Desirably the magnitude of thepotential of the pulses should be at least ten times greater than themagnitude of the continuous alternating potential. In addition, theduration of each pulse and the quiescent period between successivepulses should both extend over a time period which is at least ten timesas long as the time period required for one complete cycle of thecontinuous alternating potential. If either the pulse duration or thequiescent period between each individual pulse becomes sufiicientlyshort in time to approach the period required for one complete cycle ofthe continuous high-frequency potential, the increased light output overand above that light output to be expected is lost, as are the morerapid decay characteristics. The so-called phosphor emission decaycharacteristics represent the period of time required for the lightemission to decrease to twenty percent of the value of the maximum lightemission realized. Of course the summation of the potential magnitudesof the continuous wave and pulsed potentials used to excite the phosphorshould be less than that potential required to cause an electricalbreakdown between the electrodes. The summation of the appliedpotentials should be suflicient, however, to excite the phosphor togenerate the desired radiations.

As a specific example for operating the electroluminescent device asshown in FIG. 1, the pulse generator 22 is designed to produce squarewave potential pulses having a magnitude of volts, a pulse duration from10* to 10* second and a pulse frequency variable from 10 to 10 cyclesper second. The continuous-wave generator Ed is desi ned to produce a200 kcs. output potential, variable between 2 and 12 volts (R.M.S.). Theoutputs of the pulse generator 22 and continuous-wave generator 23 aresimultaneously applied across the electrodes 14 and 13 of theelectroluminescent element 10, or any generally-similar element in'animaging apparatus. In the following Table I, the left-hand indicates therepetition rate for the square wave pulses. The second column indicatesthe duration of each square wave pulse. The third column indicates therms. voltage of the continuous high-frequency excitation. The fourth andfifth columns indicate the percent increase or enhancement of lightoutput which is obtained when exciting the element 10 with both pulsesand continuous wave potential, as compared to the light output obtainedwhen exciting the electroluminescent element It? only with equivalentpulses. It should be noted that when an electroluminescent element isexcited with a square wave potential, a light flash will be obtainedwhen the potential is applied (rising side of the square wave) and whenthe square wave potential is removed (falling side of the square wave),as the phosphor responds to changes in the applied electric field. Thepercent enhancement of the light output obtained when the pulse isapplied is shown in column four and the percent enhancement of the lightoutput obtained when removing the pulse is shown in column five.

It should be understood that the foregoing example represents apreferred embodiment and is subject to considerable variation. Inaddition, there does not appear to be any upper limit to the frequencyof the continuous potential, except that limit imposed by capacitiveeffects due to the construction of the electroluminescent element. As anexample, if a light-transmitting, tin oxide electrode layer is used,frequencies appreciably greater than 400 kcs. may introduce excessivelosses in such a layer. Also, the pulsed potential need not be in theform of square waves, but can be any form of voltage pulses. Squarewaves are preferred, however, because of the normally rapid response anddecay of the phosphor light emission to such excitation.

The X-Y plotter as disclosed in FIG. 2 represents a conversion ofelectrical input to optical output. The

present electroluminescent device also has use in other.

optical signal by an apparatus such as described and the optical signalused to trigger a photo-transistor in order to generate a correspondingelectrical signal. Such a system achieves complete decoupling betweenthe components and can be rendered more sensitive by the use of anelectroluminescent apparatus such as described hereinbefore.

It will be recognized that the objects of the invention have beenachieved by providing an electroluminescent device which is adapted tobe operated with a pulsed energization and wherein a smaller potentialmagnitude can be used for the exciting pulses, while still achieving ahigh level of light output. In addition, there has been provided anelectroluminescent device wherein the phosphor light output will decayin a rapid fashion after the energizing potential is removed.

While best embodiments of the invention have been illustrated anddescribed hereinbefore, it is'to be particularly understood that theinvention is not limited thereto or thereby.

I claim:

1. The combination which comprises an electroluminescent element andassociated energization means: said element comprising, spacedelectrodes, and electroluminescent phosphor included between said spacedelectrodes; said energization means comprising, a first potentialgenerating means and a second potential generating means; said firstpotential generating means adapted to deliver across said spacedelectrodes a continuous alternating potential having a frequency of atleast kcs.; said second potential generating means adapted to deliveracross said spaced electrodes potential pulses having a magnitude atleast greater than the magnitude of the continuous alternating potentialadapted to be delivered across said spaced electrodes by said firstpotential generating means; the duration of each pulse and the quiescentperiod between successive pulses adapted to be delivered across saidspaced electrodes by said second potential generating means bothextending over a time period at least ten times as long as the timeperiod required for one complete cycle of the continuous alternatingpotential adapted to be delivered across said spaced electrodes by saidfirst potential generating means; the summation of potential magnitudesof the continuous and pulsed potentials adapted to be delivered by saidfirst and second potential generating means being less than thatpotential required to cause breakdown across said electrodes, butsufficient to energize said phosphor to produce radiations; and saidfirst and second potential generating means adapted to deliver theirrespective potentials simultaneously across said spaced electrodes.

2. The combination which comprises an electroluminescent element andassociated energization means: said element comprising, spacedelectrodes, and electroluminescent phosphor included between spacedelectrodes; said energization means comprising, a first potentialgenerating means and a second potential generating means; said firstpotential generating means adapted to deliver across said spacedelectrodes a continuous alternating potential having a frequency of atleast 10 kcs.; said second potential generating means adapted to deliveracross said spaced electrodes potential pulses having a magnitude atleast ten times greater than the magnitude of the continuous alternatingpotential adapted to be delivered across said spaced elecrodes by saidfirst potential generating means; the duration of each pulse and thequiescent period between successive pulses adapted to be deliveredacross said spaced electrodes by said second potential generating meansboth extending over a time period at least ten times as long as the timeperiod required for one complete cycle of the continuous alternatingpotential adapted to be delivered across said spaced electrodes by saidfirst potential generating means; the summation of potential magnitudesof the continuous and pulsed potentials adapted to be delivered by saidfirst and second potential generating means being less than thatpotential required to cause breakdown across said electrodes, butsufficient to energize said phosphor to produce visible radiations; andsaid first and second potential generating means adapted to delivertheir respective potentials simultaneously across said spacedelectrodes.

3. The combination which comprises an electroluminescent element andassociated energization means: said element comprising, spacedelectrodes, and electroluminescent phosphor included between said spacedelectrodes; said energization means comprising, a first potentialgenerating means and a second potential generating means; said firstpotential generating means adapted to deliver across said spacedelectrodes a continuous alternating potential having a frequency of 200kcs. and a magnitude of from 2 to 12 volts R.M.S.; said second potentialgenerating means adapted to deliver across said spaced electrodes aseries of potential pulses having a magnitude of about volts, arepetition rate of from 10 to 10 pulses per second, and an individualpulse duration of from 10- to l04 second; the summation of potentialmagnitudes of the continuous and pulsed potentials adapted to bedelivered by said first and second potential generating means being lessthan that potential required to cause breakdown across said electrodes,but sufficient to energize said phosphor to produce visible radiations;and said first and second potential generating means adapted to delivertheir respective potentials simultaneously across said spacedelectrodes.

4. The combination which comprises an electrolumi: nescent element andassociated energization means: said element comprising, spacedelectrodes, and electroluminescent phosphor included between said spacedelectrodes; said energization means comprising, a first potentialgenerating means and a second potential generating means; said firstpotential generating means adapted to deliver across said spacedelectrodes a continuous alternating potential having a frequency of 200kcs. and a magnitude of about 5 volts R.M.S.; said second potentialgenerating means adapted to deliver across said spaced electrodes aseries of potential pulses having a magnitude of 150 volts, a repetitionrate of 1000 pulses per second, and an individual pulse duration of 10-second; the summation of potential magnitudes of the continuous andpulsed potentials adapted to be delivered by said first and secondpotential generating means being less than that potential required tocause breakdown across said electrodes, but sufiicient to energize saidphosphor to produce visible radiations; and said first and secondpotential generating means adapted to deliver their respectivepotentials simultaneously across said spaced electrodes.

5. An electroluminescent X-Y plotter comprising: a first grid comprisingsubstantially parallel and spaced conductors; a second grid comprisingsubstantially parallel and spaced conductors separated from said firstgrid, with the axes of the conductors comprising said second gridaxially aligned at an angle to the axes of the conductors comprisingsaid first grid; material comprising electroluminescent phosphor betweensaid grids; a first potential generating means adapted to deliver at alltimes across said first grid and said second grid a continuousalternating potential having a frequency of at least 10 kcs.; a secondpotential generating means adapted to deliver potential pulses having amagnitude at least greater than the magnitude of the continuousalternating potential adapted to be delivered across said grids by saidfirst potential generating means; the duration of each pulse and thequiescent period between successive pulses adapted to be delivered bysaid second potential generating means both extending over a period oftime at least ten times as long as the time period required for onecomplete cycle of the continuous alternating potential adapted to bedelivered by said first potential generating means; the summation ofpotential magnitudes of References (Jited in the file of this patentUNITED STATES PATENTS Hanlet a- Apr. 21, 1959 Sack Dec. 15, 1959 JayApr. 12, 1960 FOREIGN PATENTS Great Britain Nov. 30, 1955

1. THE COMBINATION WHICH COMPRISES AN ELECTROLUMINESCENT ELEMENT ANDASSOCIATED ENERGIZATION MEANS: SAID ELEMENT COMPRISING, SPACEDELECTRODES, AND ELECTROLUMINESCENT PHOSPHOR INCLUDED BETWEEN SAID SPACEDELECTRODES; SAID ENERGIZATION MEANS COMPRISING, A FIRST POTENTIALGENERATING MEANS AND A SECOND POTENTIAL GENERATING MEANS; SAID FIRSTPOTENTIAL GENERATING MEANS ADAPTED TO DELIVER ACROSS SAID SPACEDELECTRODES A CONTINUOUS ALTERNATING POTENTIAL HAVING A FREQUENCY OF ATLEAST 10 KCS.; SAID SECOND POTENTIAL GENERATING MEANS ADAPTED TO DELIVERACROSS SAID SPACED ELECTRODES POTENTIAL PULSES HAVING A MAGNITUDE ATLEAST GREATER THAN THE MAGNITUDE OF THE CONTINUOUS ALTERNATING POTENTIALADAPTED TO BE DELIVERED ACROSS SAID SPACED ELECTRODES BY SAID FIRSTPOTENTIAL GENERATING MEANS; THE DURATION OF EACH PULSE AND THE QUIESCENTPERIOD BETWEEN SUCESSIVE PULSES ADAPTED TO BE DELIVERED ACROSS SAIDSPACED ELECTRODES BY SAID SECOND POTENTIAL GENERATING MEANS BOTHEXTENDING OVER A TIME PERIOD AT LEAST TEN TIMES AS LONG AS THE TIMEPERIOD REQUIRED FOR ONE COMPLETE CYCLE OF THE CONTINUOUS ALTERNATINGPOTENTIAL ADAPTED TO BE DELIVERED ACROSS SAID SPACED ELECTRODES BY SAIDFIRST POTENTIAL GENERATING MEANS; THE SUMMATION OF POTENTIAL MAGNITUDESOF THE CONTINUOUS AND PULSED POTENTIALS ADAPTED TO BE DELIVERED BY SAIDFIRST AND SECOND POTENTIAL GENERATING MEANS BEING LESS THAN THATPOTENTIAL REQUIRED TO CAUSE BREAKDOWN ACROSS SAID ELECTRODES, BUTSUFFICIENT TO ENERGIZE SAID PHOSPHOR TO PRODUCE RADIATIONS; AND SAIDFIRST AND SECOND POTENTIAL GENERATING MEANS ADAPTED TO DELIVER THEIRRESPECTIVE POTENTIALS SIMULTANEOUSLY ACROSS SAID SPACED ELECTRODES.